The present invention relates to the transmission of mechanical power, especially to transmission systems of the flexible belt type, including both single belt and multiple belt transmission systems. More particularly the invention relates to an apparatus for controlling the tension of such belts.
Flexible drive belts are widely used for transmitting power from large engines to machinery being driven by such engines. When a belt stretches sufficiently, the belt begins to slip and full power is not transmitted from the driving pulley to the driven pulley. As the belt begins to slip, friction causes the temperature of the belt to increase, and the rate of deterioration increases rapidly after slipping begins.
Both single drive belts and multiple drive belts have similar slipping problems. In a single drive belt application, as the belt begins to slip, increased tension must be placed on the belt by means of an idler pulley connected to some fixed structure such as the driving engine, for example, or by increasing the distance of the drive pulley from the driven pulley. If such steps are not taken the belt must be removed and replaced with a new belt.
It is well known that for many purposes multiple belt drive offers a large number of advantages as compared to single belt drives, especially in large power installations for which the advantages are so well understood to require no discussion here. With the introduction of multiple belt transmission of power there appeared a problem which did not exist with a single belt type of transmission, namely that of keeping all the belts under equal tension. This problem obviously increases with the number of belts because it is extremely difficult to secure several belts of the same length which will maintain the same length throughout their life. It is apparent that if the belts differ in length they will not carry equal fractions of power transmitted with the result that some of them will be overloaded and will wear out prematurely. Another disadvantage from the unequal loading of the belts is that the sheaves or pulleys of the prime mover or driving apparatus and of the driven apparatus will be subjected to uneven strains which may injure the shafts and the bearings. Furthermore, the whipping of the partially loaded belts causes vibration and excessive wear both on the belt, and the bearings as well, resulting in inefficient power transmission. It will therefore be apparent that in large installations where multiple belts are used, in which case shut-downs are very expensive, it is especially important that as many as possible of these and other attendant disadvantages be avoided.
Such shut-downs are expensive for several reasons. In multiple belt transmission of power, when one of the belts start to slip sufficiently to cause a power loss, it is commonly necessary to change every belt in the system rather than just the loose belt. All the belts are changed because the belts not slipping have stretched to some extent due to previous wear, and a new belt would not be the same length as the older belts. Thus, the cost of the belts themselves is a great expense. In addition, the increased time for changing multiple belts increases expenses due to loss of the use of the driven equipment.
Belt tensioning devices are known in the art for both single and multiple power transmission. U.S. Pat. Nos. 3,837,291; 3,391,807; 2,499,287; 2,213,992; 2,726,364; and 2,066,721 disclose various single and multiple belt tensioning devices.
However, the devices of the prior art require that the tensioning apparatus be attached to some structure adjacent to or connected to the prime mover or the driven apparatus to position the tensioning apparatus properly over the appropriate belt or belts. Such attachment results in significant disadvantages. It is difficult to place a safety guard over an arm or member attaching an idler pulley and over the belts. Furthermore, the structure necessary to stationarily position the belt tensioning apparatus over the belts can be bulky and expensive to manufacture.